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GNU Offloading and Multi Processing Runtime LibraryThe GNU OpenMP and OpenACC Implementation

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Copyright c© 2006-2017 Free Software Foundation, Inc.Permission is granted to copy, distribute and/or modify this document under the termsof the GNU Free Documentation License, Version 1.3 or any later version published bythe Free Software Foundation; with the Invariant Sections being “Funding Free Software”,the Front-Cover texts being (a) (see below), and with the Back-Cover Texts being (b) (seebelow). A copy of the license is included in the section entitled “GNU Free DocumentationLicense”.(a) The FSF’s Front-Cover Text is:A GNU Manual(b) The FSF’s Back-Cover Text is:You have freedom to copy and modify this GNU Manual, like GNU software. Copiespublished by the Free Software Foundation raise funds for GNU development.

i

Short Contents

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1 Enabling OpenMP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2 Runtime Library Routines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3 Environment Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

4 Enabling OpenACC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

5 OpenACC Runtime Library Routines . . . . . . . . . . . . . . . . . . . . 27

6 OpenACC Environment Variables . . . . . . . . . . . . . . . . . . . . . . . 39

7 CUDA Streams Usage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

8 OpenACC Library Interoperability . . . . . . . . . . . . . . . . . . . . . . 43

9 The libgomp ABI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

10 Reporting Bugs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

GNU General Public License . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

GNU Free Documentation License . . . . . . . . . . . . . . . . . . . . . . . . . . 67

Funding Free Software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

Library Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

iii

Table of Contents

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1 Enabling OpenMP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2 Runtime Library Routines . . . . . . . . . . . . . . . . . . . . . 52.1 omp_get_active_level – Number of parallel regions . . . . . . . . . . . 52.2 omp_get_ancestor_thread_num – Ancestor thread ID . . . . . . . . . . 52.3 omp_get_cancellation – Whether cancellation support is enabled

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.4 omp_get_default_device – Get the default device for target

regions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62.5 omp_get_dynamic – Dynamic teams setting . . . . . . . . . . . . . . . . . . . . . 62.6 omp_get_level – Obtain the current nesting level . . . . . . . . . . . . . . 62.7 omp_get_max_active_levels – Maximum number of active

regions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72.8 omp_get_max_task_priority – Maximum priority value . . . . . . . . 72.9 omp_get_max_threads – Maximum number of threads of parallel

region . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72.10 omp_get_nested – Nested parallel regions . . . . . . . . . . . . . . . . . . . . . 82.11 omp_get_num_devices – Number of target devices . . . . . . . . . . . . . 82.12 omp_get_num_procs – Number of processors online . . . . . . . . . . . . 82.13 omp_get_num_teams – Number of teams . . . . . . . . . . . . . . . . . . . . . . . 92.14 omp_get_num_threads – Size of the active team . . . . . . . . . . . . . . . 92.15 omp_get_proc_bind – Whether theads may be moved between

CPUs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92.16 omp_get_schedule – Obtain the runtime scheduling method . . 102.17 omp_get_team_num – Get team number . . . . . . . . . . . . . . . . . . . . . . 102.18 omp_get_team_size – Number of threads in a team . . . . . . . . . . 102.19 omp_get_thread_limit – Maximum number of threads . . . . . . 112.20 omp_get_thread_num – Current thread ID . . . . . . . . . . . . . . . . . . . 112.21 omp_in_parallel – Whether a parallel region is active . . . . . . . 112.22 omp_in_final – Whether in final or included task region . . . . . 122.23 omp_is_initial_device – Whether executing on the host device

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122.24 omp_set_default_device – Set the default device for target

regions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122.25 omp_set_dynamic – Enable/disable dynamic teams . . . . . . . . . . . 132.26 omp_set_max_active_levels – Limits the number of active

parallel regions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132.27 omp_set_nested – Enable/disable nested parallel regions . . . . . 132.28 omp_set_num_threads – Set upper team size limit . . . . . . . . . . . . 142.29 omp_set_schedule – Set the runtime scheduling method . . . . . 14

iv GNU libgomp

2.30 omp_init_lock – Initialize simple lock . . . . . . . . . . . . . . . . . . . . . . . 142.31 omp_set_lock – Wait for and set simple lock . . . . . . . . . . . . . . . . . 152.32 omp_test_lock – Test and set simple lock if available . . . . . . . . 152.33 omp_unset_lock – Unset simple lock . . . . . . . . . . . . . . . . . . . . . . . . . 152.34 omp_destroy_lock – Destroy simple lock . . . . . . . . . . . . . . . . . . . . . 162.35 omp_init_nest_lock – Initialize nested lock . . . . . . . . . . . . . . . . . 162.36 omp_set_nest_lock – Wait for and set nested lock . . . . . . . . . . . 162.37 omp_test_nest_lock – Test and set nested lock if available . . 172.38 omp_unset_nest_lock – Unset nested lock . . . . . . . . . . . . . . . . . . . 172.39 omp_destroy_nest_lock – Destroy nested lock . . . . . . . . . . . . . . . 182.40 omp_get_wtick – Get timer precision . . . . . . . . . . . . . . . . . . . . . . . . 182.41 omp_get_wtime – Elapsed wall clock time . . . . . . . . . . . . . . . . . . . . 18

3 Environment Variables . . . . . . . . . . . . . . . . . . . . . . . . 193.1 OMP_CANCELLATION – Set whether cancellation is activated . . . . . 193.2 OMP_DISPLAY_ENV – Show OpenMP version and environment

variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193.3 OMP_DEFAULT_DEVICE – Set the device used in target regions . . . 193.4 OMP_DYNAMIC – Dynamic adjustment of threads . . . . . . . . . . . . . . . . 193.5 OMP_MAX_ACTIVE_LEVELS – Set the maximum number of nested

parallel regions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203.6 OMP_MAX_TASK_PRIORITY – Set the maximum priority . . . . . . . . . 203.7 OMP_NESTED – Nested parallel regions . . . . . . . . . . . . . . . . . . . . . . . . . . 203.8 OMP_NUM_THREADS – Specifies the number of threads to use . . . . . 203.9 OMP_PROC_BIND – Whether theads may be moved between CPUs

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203.10 OMP_PLACES – Specifies on which CPUs the theads should be

placed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213.11 OMP_STACKSIZE – Set default thread stack size . . . . . . . . . . . . . . . . 223.12 OMP_SCHEDULE – How threads are scheduled . . . . . . . . . . . . . . . . . . 223.13 OMP_THREAD_LIMIT – Set the maximum number of threads . . . . 223.14 OMP_WAIT_POLICY – How waiting threads are handled . . . . . . . . 223.15 GOMP_CPU_AFFINITY – Bind threads to specific CPUs . . . . . . . . . 233.16 GOMP_DEBUG – Enable debugging output . . . . . . . . . . . . . . . . . . . . . . 233.17 GOMP_STACKSIZE – Set default thread stack size . . . . . . . . . . . . . . 233.18 GOMP_SPINCOUNT – Set the busy-wait spin count . . . . . . . . . . . . . . 243.19 GOMP_RTEMS_THREAD_POOLS – Set the RTEMS specific thread

pools . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

4 Enabling OpenACC . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

v

5 OpenACC Runtime Library Routines . . . . . . . 275.1 acc_get_num_devices – Get number of devices for given device

type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 275.2 acc_set_device_type – Set type of device accelerator to use. . . 275.3 acc_get_device_type – Get type of device accelerator to be used.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 275.4 acc_set_device_num – Set device number to use. . . . . . . . . . . . . . 285.5 acc_get_device_num – Get device number to be used. . . . . . . . . 285.6 acc_async_test – Test for completion of a specific asynchronous

operation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 285.7 acc_async_test_all – Tests for completion of all asynchronous

operations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 295.8 acc_wait – Wait for completion of a specific asynchronous

operation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 295.9 acc_wait_all – Waits for completion of all asynchronous

operations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 295.10 acc_wait_all_async – Wait for completion of all asynchronous

operations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 305.11 acc_wait_async – Wait for completion of asynchronous

operations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 305.12 acc_init – Initialize runtime for a specific device type. . . . . . . . 305.13 acc_shutdown – Shuts down the runtime for a specific device

type. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 305.14 acc_on_device – Whether executing on a particular device . . . 315.15 acc_malloc – Allocate device memory. . . . . . . . . . . . . . . . . . . . . . . . 315.16 acc_free – Free device memory. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315.17 acc_copyin – Allocate device memory and copy host memory to

it. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 325.18 acc_present_or_copyin – If the data is not present on the

device, allocate device memory and copy from host memory. . . . . 325.19 acc_create – Allocate device memory and map it to host

memory. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 335.20 acc_present_or_create – If the data is not present on the

device, allocate device memory and map it to host memory. . . . . 335.21 acc_copyout – Copy device memory to host memory. . . . . . . . . 345.22 acc_delete – Free device memory. . . . . . . . . . . . . . . . . . . . . . . . . . . . 345.23 acc_update_device – Update device memory from mapped host

memory. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 355.24 acc_update_self – Update host memory from mapped device

memory. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 355.25 acc_map_data – Map previously allocated device memory to host

memory. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 365.26 acc_unmap_data – Unmap device memory from host memory.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 365.27 acc_deviceptr – Get device pointer associated with specific host

address. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 365.28 acc_hostptr – Get host pointer associated with specific device

address. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

vi GNU libgomp

5.29 acc_is_present – Indicate whether host variable / array ispresent on device. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

5.30 acc_memcpy_to_device – Copy host memory to device memory.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

5.31 acc_memcpy_from_device – Copy device memory to hostmemory. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

5.32 acc_get_current_cuda_device – Get CUDA device handle.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

5.33 acc_get_current_cuda_context – Get CUDA context handle.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

5.34 acc_get_cuda_stream – Get CUDA stream handle. . . . . . . . . . . 385.35 acc_set_cuda_stream – Set CUDA stream handle. . . . . . . . . . . . 38

6 OpenACC Environment Variables . . . . . . . . . . . 396.1 ACC_DEVICE_TYPE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 396.2 ACC_DEVICE_NUM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 396.3 GCC_ACC_NOTIFY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

7 CUDA Streams Usage . . . . . . . . . . . . . . . . . . . . . . . . 41

8 OpenACC Library Interoperability . . . . . . . . . . 438.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 438.2 First invocation: NVIDIA CUBLAS library API . . . . . . . . . . . . . . . 438.3 First invocation: OpenACC library API . . . . . . . . . . . . . . . . . . . . . . . 448.4 OpenACC library and environment variables . . . . . . . . . . . . . . . . . . 45

9 The libgomp ABI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 479.1 Implementing MASTER construct . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 479.2 Implementing CRITICAL construct . . . . . . . . . . . . . . . . . . . . . . . . . . . 479.3 Implementing ATOMIC construct . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 479.4 Implementing FLUSH construct . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 479.5 Implementing BARRIER construct . . . . . . . . . . . . . . . . . . . . . . . . . . . . 479.6 Implementing THREADPRIVATE construct . . . . . . . . . . . . . . . . . . 479.7 Implementing PRIVATE clause . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 489.8 Implementing FIRSTPRIVATE LASTPRIVATE COPYIN and

COPYPRIVATE clauses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 489.9 Implementing REDUCTION clause . . . . . . . . . . . . . . . . . . . . . . . . . . . . 489.10 Implementing PARALLEL construct . . . . . . . . . . . . . . . . . . . . . . . . . 489.11 Implementing FOR construct . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 499.12 Implementing ORDERED construct . . . . . . . . . . . . . . . . . . . . . . . . . . 509.13 Implementing SECTIONS construct . . . . . . . . . . . . . . . . . . . . . . . . . . 509.14 Implementing SINGLE construct . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 509.15 Implementing OpenACC’s PARALLEL construct . . . . . . . . . . . . . 51

10 Reporting Bugs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

vii

GNU General Public License . . . . . . . . . . . . . . . . . . . . . 55

GNU Free Documentation License . . . . . . . . . . . . . . . 67ADDENDUM: How to use this License for your documents . . . . . . . . . 74

Funding Free Software . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

Library Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

Introduction 1

Introduction

This manual documents the usage of libgomp, the GNU Offloading and Multi Process-ing Runtime Library. This includes the GNU implementation of the OpenMP ApplicationProgramming Interface (API) for multi-platform shared-memory parallel programming inC/C++ and Fortran, and the GNU implementation of the OpenACC Application Program-ming Interface (API) for offloading of code to accelerator devices in C/C++ and Fortran.

Originally, libgomp implemented the GNU OpenMP Runtime Library. Based on this,support for OpenACC and offloading (both OpenACC and OpenMP 4’s target construct)has been added later on, and the library’s name changed to GNU Offloading and MultiProcessing Runtime Library.

http://www.openmp.org

http://www.openacc.org/

Chapter 1: Enabling OpenMP 3

1 Enabling OpenMP

To activate the OpenMP extensions for C/C++ and Fortran, the compile-time flag -fopenmpmust be specified. This enables the OpenMP directive #pragma omp in C/C++ and !$ompdirectives in free form, c$omp, *$omp and !$omp directives in fixed form, !$ conditionalcompilation sentinels in free form and c$, *$ and !$ sentinels in fixed form, for Fortran.The flag also arranges for automatic linking of the OpenMP runtime library (Chapter 2[Runtime Library Routines], page 5).

A complete description of all OpenMP directives accepted may be found in the OpenMPApplication Program Interface manual, version 4.5.



Chapter 2: Runtime Library Routines 5

2 Runtime Library Routines

The runtime routines described here are defined by Section 3 of the OpenMP specificationin version 4.5. The routines are structured in following three parts:

2.1 omp_get_active_level – Number of parallel regions

Description:This function returns the nesting level for the active parallel blocks, whichenclose the calling call.

C/C++Prototype: int omp_get_active_level(void);

Fortran:Interface: integer function omp_get_active_level()

See also: Section 2.6 [omp get level], page 6, Section 2.7 [omp get max active levels],page 7, Section 2.26 [omp set max active levels], page 13

Reference: OpenMP specification v4.5, Section 3.2.20.

2.2 omp_get_ancestor_thread_num – Ancestor thread ID

Description:This function returns the thread identification number for the given nestinglevel of the current thread. For values of level outside zero to omp_get_level-1 is returned; if level is omp_get_level the result is identical to omp_get_thread_num.

C/C++Prototype: int omp_get_ancestor_thread_num(int level);

Fortran:Interface: integer function omp_get_ancestor_thread_num(level)

integer level

See also: Section 2.6 [omp get level], page 6, Section 2.20 [omp get thread num],page 11, Section 2.18 [omp get team size], page 10


2.3 omp_get_cancellation – Whether cancellation support isenabled

Description:This function returns true if cancellation is activated, false otherwise. Here,true and false represent their language-specific counterparts. Unless OMP_CANCELLATION is set true, cancellations are deactivated.

C/C++:Prototype: int omp_get_cancellation(void);

http://www.openmp.org/


6 GNU libgomp

Fortran:Interface: logical function omp_get_cancellation()

See also: Section 3.1 [OMP CANCELLATION], page 19


2.4 omp_get_default_device – Get the default device fortarget regions

Description:Get the default device for target regions without device clause.

C/C++:Prototype: int omp_get_default_device(void);

Fortran:Interface: integer function omp_get_default_device()

See also: Section 3.3 [OMP DEFAULT DEVICE], page 19, Section 2.24[omp set default device], page 12


2.5 omp_get_dynamic – Dynamic teams setting

Description:This function returns true if enabled, false otherwise. Here, true and falserepresent their language-specific counterparts.The dynamic team setting may be initialized at startup by the OMP_DYNAMICenvironment variable or at runtime using omp_set_dynamic. If undefined, dy-namic adjustment is disabled by default.

C/C++:Prototype: int omp_get_dynamic(void);

Fortran:Interface: logical function omp_get_dynamic()

See also: Section 2.25 [omp set dynamic], page 13, Section 3.4 [OMP DYNAMIC],page 19


2.6 omp_get_level – Obtain the current nesting level

Description:This function returns the nesting level for the parallel blocks, which enclose thecalling call.

C/C++Prototype: int omp_get_level(void);





Fortran:Interface: integer function omp_level()

See also: Section 2.1 [omp get active level], page 5


2.7 omp_get_max_active_levels – Maximum number of activeregions

Description:This function obtains the maximum allowed number of nested, active parallelregions.

C/C++Prototype: int omp_get_max_active_levels(void);

Fortran:Interface: integer function omp_get_max_active_levels()

See also: Section 2.26 [omp set max active levels], page 13, Section 2.1[omp get active level], page 5


2.8 omp_get_max_task_priority – Maximum priority value

that can be set for tasks.

Description:This function obtains the maximum allowed priority number for tasks.

C/C++Prototype: int omp_get_max_task_priority(void);

Fortran:Interface: integer function omp_get_max_task_priority()


2.9 omp_get_max_threads – Maximum number of threads ofparallel region

Description:Return the maximum number of threads used for the current parallel regionthat does not use the clause num_threads.

C/C++:Prototype: int omp_get_max_threads(void);

Fortran:Interface: integer function omp_get_max_threads()




8 GNU libgomp

See also: Section 2.28 [omp set num threads], page 14, Section 2.25 [omp set dynamic],page 13, Section 2.19 [omp get thread limit], page 11


2.10 omp_get_nested – Nested parallel regions

Description:This function returns true if nested parallel regions are enabled, false other-wise. Here, true and false represent their language-specific counterparts.

Nested parallel regions may be initialized at startup by the OMP_NESTED envi-ronment variable or at runtime using omp_set_nested. If undefined, nestedparallel regions are disabled by default.

C/C++:

Prototype: int omp_get_nested(void);

Fortran:

Interface: logical function omp_get_nested()

See also: Section 2.27 [omp set nested], page 13, Section 3.7 [OMP NESTED], page 20


2.11 omp_get_num_devices – Number of target devices

Description:Returns the number of target devices.

C/C++:

Prototype: int omp_get_num_devices(void);

Fortran:

Interface: integer function omp_get_num_devices()


2.12 omp_get_num_procs – Number of processors online

Description:Returns the number of processors online on that device.

C/C++:

Prototype: int omp_get_num_procs(void);

Fortran:

Interface: integer function omp_get_num_procs()







2.13 omp_get_num_teams – Number of teams

Description:Returns the number of teams in the current team region.

C/C++:Prototype: int omp_get_num_teams(void);

Fortran:Interface: integer function omp_get_num_teams()


2.14 omp_get_num_threads – Size of the active team

Description:Returns the number of threads in the current team. In a sequential section ofthe program omp_get_num_threads returns 1.The default team size may be initialized at startup by the OMP_NUM_THREADSenvironment variable. At runtime, the size of the current team may be seteither by the NUM_THREADS clause or by omp_set_num_threads. If none of theabove were used to define a specific value and OMP_DYNAMIC is disabled, onethread per CPU online is used.

C/C++:Prototype: int omp_get_num_threads(void);

Fortran:Interface: integer function omp_get_num_threads()

See also: Section 2.9 [omp get max threads], page 7, Section 2.28 [omp set num threads],page 14, Section 3.8 [OMP NUM THREADS], page 20


2.15 omp_get_proc_bind – Whether theads may be movedbetween CPUs

Description:This functions returns the currently active thread affinity policy, which is setvia OMP_PROC_BIND. Possible values are omp_proc_bind_false, omp_proc_bind_true, omp_proc_bind_master, omp_proc_bind_close and omp_proc_bind_spread.

C/C++:Prototype: omp_proc_bind_t omp_get_proc_bind(void);

Fortran:Interface: integer(kind=omp_proc_bind_kind) function omp_get_proc_

bind()



10 GNU libgomp

See also: Section 3.9 [OMP PROC BIND], page 20, Section 3.10 [OMP PLACES],page 21, Section 3.15 [GOMP CPU AFFINITY], page 23,


2.16 omp_get_schedule – Obtain the runtime schedulingmethod

Description:Obtain the runtime scheduling method. The kind argument will be set tothe value omp_sched_static, omp_sched_dynamic, omp_sched_guided or omp_sched_auto. The second argument, chunk size, is set to the chunk size.

C/C++Prototype: void omp_get_schedule(omp_sched_t *kind, int *chunk_size);

Fortran:Interface: subroutine omp_get_schedule(kind, chunk_size)

integer(kind=omp_sched_kind) kindinteger chunk_size

See also: Section 2.29 [omp set schedule], page 14, Section 3.12 [OMP SCHEDULE],page 22


2.17 omp_get_team_num – Get team number

Description:Returns the team number of the calling thread.

C/C++:Prototype: int omp_get_team_num(void);

Fortran:Interface: integer function omp_get_team_num()


2.18 omp_get_team_size – Number of threads in a team

Description:This function returns the number of threads in a thread team to which eitherthe current thread or its ancestor belongs. For values of level outside zero toomp_get_level, -1 is returned; if level is zero, 1 is returned, and for omp_get_level, the result is identical to omp_get_num_threads.

C/C++:Prototype: int omp_get_team_size(int level);

Fortran:Interface: integer function omp_get_team_size(level)

integer level





See also: Section 2.14 [omp get num threads], page 9, Section 2.6 [omp get level], page 6,Section 2.2 [omp get ancestor thread num], page 5


2.19 omp_get_thread_limit – Maximum number of threads

Description:Return the maximum number of threads of the program.

C/C++:Prototype: int omp_get_thread_limit(void);

Fortran:Interface: integer function omp_get_thread_limit()

See also: Section 2.9 [omp get max threads], page 7, Section 3.13 [OMP THREAD LIMIT],page 22


2.20 omp_get_thread_num – Current thread ID

Description:Returns a unique thread identification number within the current team. In asequential parts of the program, omp_get_thread_num always returns 0. Inparallel regions the return value varies from 0 to omp_get_num_threads-1 in-clusive. The return value of the master thread of a team is always 0.

C/C++:Prototype: int omp_get_thread_num(void);

Fortran:Interface: integer function omp_get_thread_num()

See also: Section 2.14 [omp get num threads], page 9, Section 2.2 [omp get ancestor thread num],page 5


2.21 omp_in_parallel – Whether a parallel region is active

Description:This function returns true if currently running in parallel, false otherwise.Here, true and false represent their language-specific counterparts.

C/C++:Prototype: int omp_in_parallel(void);

Fortran:Interface: logical function omp_in_parallel()






12 GNU libgomp

2.22 omp_in_final – Whether in final or included task region

Description:This function returns true if currently running in a final or included task re-gion, false otherwise. Here, true and false represent their language-specificcounterparts.

C/C++:

Prototype: int omp_in_final(void);

Fortran:

Interface: logical function omp_in_final()


2.23 omp_is_initial_device – Whether executing on the hostdevice

Description:This function returns true if currently running on the host device, false oth-erwise. Here, true and false represent their language-specific counterparts.

C/C++:

Prototype: int omp_is_initial_device(void);

Fortran:

Interface: logical function omp_is_initial_device()


2.24 omp_set_default_device – Set the default device fortarget regions

Description:Set the default device for target regions without device clause. The argumentshall be a nonnegative device number.

C/C++:

Prototype: void omp_set_default_device(int device_num);

Fortran:

Interface: subroutine omp_set_default_device(device_num)integer device_num

See also: Section 3.3 [OMP DEFAULT DEVICE], page 19, Section 2.4[omp get default device], page 6






2.25 omp_set_dynamic – Enable/disable dynamic teams

Description:Enable or disable the dynamic adjustment of the number of threads within ateam. The function takes the language-specific equivalent of true and false,where true enables dynamic adjustment of team sizes and false disables it.

C/C++:Prototype: void omp_set_dynamic(int dynamic_threads);

Fortran:Interface: subroutine omp_set_dynamic(dynamic_threads)

logical, intent(in) :: dynamic_threads

See also: Section 3.4 [OMP DYNAMIC], page 19, Section 2.5 [omp get dynamic], page 6


2.26 omp_set_max_active_levels – Limits the number ofactive parallel regions

Description:This function limits the maximum allowed number of nested, active parallelregions.

C/C++Prototype: void omp_set_max_active_levels(int max_levels);

Fortran:Interface: subroutine omp_set_max_active_levels(max_levels)

integer max_levels

See also: Section 2.7 [omp get max active levels], page 7, Section 2.1[omp get active level], page 5


2.27 omp_set_nested – Enable/disable nested parallel regions

Description:Enable or disable nested parallel regions, i.e., whether team members are al-lowed to create new teams. The function takes the language-specific equivalentof true and false, where true enables dynamic adjustment of team sizes andfalse disables it.

C/C++:Prototype: void omp_set_nested(int nested);

Fortran:Interface: subroutine omp_set_nested(nested)

logical, intent(in) :: nested

See also: Section 3.7 [OMP NESTED], page 20, Section 2.10 [omp get nested], page 8





14 GNU libgomp

2.28 omp_set_num_threads – Set upper team size limit

Description:Specifies the number of threads used by default in subsequent parallel sections,if those do not specify a num_threads clause. The argument of omp_set_num_threads shall be a positive integer.

C/C++:Prototype: void omp_set_num_threads(int num_threads);

Fortran:Interface: subroutine omp_set_num_threads(num_threads)

integer, intent(in) :: num_threads

See also: Section 3.8 [OMP NUM THREADS], page 20, Section 2.14[omp get num threads], page 9, Section 2.9 [omp get max threads],page 7


2.29 omp_set_schedule – Set the runtime scheduling method

Description:Sets the runtime scheduling method. The kind argument can have thevalue omp_sched_static, omp_sched_dynamic, omp_sched_guided oromp_sched_auto. Except for omp_sched_auto, the chunk size is set to thevalue of chunk size if positive, or to the default value if zero or negative. Foromp_sched_auto the chunk size argument is ignored.

C/C++Prototype: void omp_set_schedule(omp_sched_t kind, int chunk_size);

Fortran:Interface: subroutine omp_set_schedule(kind, chunk_size)

integer(kind=omp_sched_kind) kindinteger chunk_size

See also: Section 2.16 [omp get schedule], page 10 Section 3.12 [OMP SCHEDULE],page 22


2.30 omp_init_lock – Initialize simple lock

Description:Initialize a simple lock. After initialization, the lock is in an unlocked state.

C/C++:Prototype: void omp_init_lock(omp_lock_t *lock);

Fortran:Interface: subroutine omp_init_lock(svar)

integer(omp_lock_kind), intent(out) :: svar




See also: Section 2.34 [omp destroy lock], page 16


2.31 omp_set_lock – Wait for and set simple lock

Description:Before setting a simple lock, the lock variable must be initialized by omp_init_lock. The calling thread is blocked until the lock is available. If the lock isalready held by the current thread, a deadlock occurs.

C/C++:Prototype: void omp_set_lock(omp_lock_t *lock);

Fortran:Interface: subroutine omp_set_lock(svar)

integer(omp_lock_kind), intent(inout) :: svar

See also: Section 2.30 [omp init lock], page 14, Section 2.32 [omp test lock], page 15,Section 2.33 [omp unset lock], page 15


2.32 omp_test_lock – Test and set simple lock if available

Description:Before setting a simple lock, the lock variable must be initialized by omp_init_lock. Contrary to omp_set_lock, omp_test_lock does not block if the lock isnot available. This function returns true upon success, false otherwise. Here,true and false represent their language-specific counterparts.

C/C++:Prototype: int omp_test_lock(omp_lock_t *lock);

Fortran:Interface: logical function omp_test_lock(svar)


See also: Section 2.30 [omp init lock], page 14, Section 2.31 [omp set lock], page 15,Section 2.31 [omp set lock], page 15


2.33 omp_unset_lock – Unset simple lock

Description:A simple lock about to be unset must have been locked by omp_set_lock oromp_test_lock before. In addition, the lock must be held by the thread callingomp_unset_lock. Then, the lock becomes unlocked. If one or more threadsattempted to set the lock before, one of them is chosen to, again, set the lockto itself.




16 GNU libgomp

C/C++:Prototype: void omp_unset_lock(omp_lock_t *lock);

Fortran:Interface: subroutine omp_unset_lock(svar)


See also: Section 2.31 [omp set lock], page 15, Section 2.32 [omp test lock], page 15


2.34 omp_destroy_lock – Destroy simple lock

Description:Destroy a simple lock. In order to be destroyed, a simple lock must be in theunlocked state.

C/C++:Prototype: void omp_destroy_lock(omp_lock_t *lock);

Fortran:Interface: subroutine omp_destroy_lock(svar)


See also: Section 2.30 [omp init lock], page 14


2.35 omp_init_nest_lock – Initialize nested lock

Description:Initialize a nested lock. After initialization, the lock is in an unlocked state andthe nesting count is set to zero.

C/C++:Prototype: void omp_init_nest_lock(omp_nest_lock_t *lock);

Fortran:Interface: subroutine omp_init_nest_lock(nvar)

integer(omp_nest_lock_kind), intent(out) :: nvar

See also: Section 2.39 [omp destroy nest lock], page 18


2.36 omp_set_nest_lock – Wait for and set nested lock

Description:Before setting a nested lock, the lock variable must be initialized by omp_init_nest_lock. The calling thread is blocked until the lock is available. If thelock is already held by the current thread, the nesting count for the lock isincremented.





C/C++:Prototype: void omp_set_nest_lock(omp_nest_lock_t *lock);

Fortran:Interface: subroutine omp_set_nest_lock(nvar)

integer(omp_nest_lock_kind), intent(inout) :: nvar

See also: Section 2.35 [omp init nest lock], page 16, Section 2.38 [omp unset nest lock],page 17


2.37 omp_test_nest_lock – Test and set nested lock ifavailable

Description:Before setting a nested lock, the lock variable must be initialized by omp_init_nest_lock. Contrary to omp_set_nest_lock, omp_test_nest_lock does notblock if the lock is not available. If the lock is already held by the currentthread, the new nesting count is returned. Otherwise, the return value equalszero.

C/C++:Prototype: int omp_test_nest_lock(omp_nest_lock_t *lock);

Fortran:Interface: logical function omp_test_nest_lock(nvar)


See also: Section 2.30 [omp init lock], page 14, Section 2.31 [omp set lock], page 15,Section 2.31 [omp set lock], page 15


2.38 omp_unset_nest_lock – Unset nested lock

Description:A nested lock about to be unset must have been locked by omp_set_nested_lock or omp_test_nested_lock before. In addition, the lock must be held bythe thread calling omp_unset_nested_lock. If the nesting count drops to zero,the lock becomes unlocked. If one ore more threads attempted to set the lockbefore, one of them is chosen to, again, set the lock to itself.

C/C++:Prototype: void omp_unset_nest_lock(omp_nest_lock_t *lock);

Fortran:Interface: subroutine omp_unset_nest_lock(nvar)


See also: Section 2.36 [omp set nest lock], page 16





18 GNU libgomp

2.39 omp_destroy_nest_lock – Destroy nested lock

Description:Destroy a nested lock. In order to be destroyed, a nested lock must be in theunlocked state and its nesting count must equal zero.

C/C++:Prototype: void omp_destroy_nest_lock(omp_nest_lock_t *);

Fortran:Interface: subroutine omp_destroy_nest_lock(nvar)


See also: Section 2.30 [omp init lock], page 14


2.40 omp_get_wtick – Get timer precision

Description:Gets the timer precision, i.e., the number of seconds between two successiveclock ticks.

C/C++:Prototype: double omp_get_wtick(void);

Fortran:Interface: double precision function omp_get_wtick()

See also: Section 2.41 [omp get wtime], page 18


2.41 omp_get_wtime – Elapsed wall clock time

Description:Elapsed wall clock time in seconds. The time is measured per thread, no guar-antee can be made that two distinct threads measure the same time. Time ismeasured from some "time in the past", which is an arbitrary time guaranteednot to change during the execution of the program.

C/C++:Prototype: double omp_get_wtime(void);

Fortran:Interface: double precision function omp_get_wtime()

See also: Section 2.40 [omp get wtick], page 18





Chapter 3: Environment Variables 19

3 Environment Variables

The environment variables which beginning with OMP_ are defined by section 4 of theOpenMP specification in version 4.5, while those beginning with GOMP_ are GNU exten-sions.

3.1 OMP_CANCELLATION – Set whether cancellation is activated

Description:If set to TRUE, the cancellation is activated. If set to FALSE or if unset, cancel-lation is disabled and the cancel construct is ignored.

See also: Section 2.3 [omp get cancellation], page 5

Reference: OpenMP specification v4.5, Section 4.11

3.2 OMP_DISPLAY_ENV – Show OpenMP version andenvironment variables

Description:If set to TRUE, the OpenMP version number and the values associated withthe OpenMP environment variables are printed to stderr. If set to VERBOSE,it additionally shows the value of the environment variables which are GNUextensions. If undefined or set to FALSE, this information will not be shown.


3.3 OMP_DEFAULT_DEVICE – Set the device used in targetregions

Description:Set to choose the device which is used in a target region, unless the value isoverridden by omp_set_default_device or by a device clause. The value shallbe the nonnegative device number. If no device with the given device numberexists, the code is executed on the host. If unset, device number 0 will be used.

See also: Section 2.4 [omp get default device], page 6, Section 2.24[omp set default device], page 12,


3.4 OMP_DYNAMIC – Dynamic adjustment of threads

Description:Enable or disable the dynamic adjustment of the number of threads withina team. The value of this environment variable shall be TRUE or FALSE. Ifundefined, dynamic adjustment is disabled by default.

See also: Section 2.25 [omp set dynamic], page 13






20 GNU libgomp

3.5 OMP_MAX_ACTIVE_LEVELS – Set the maximum number ofnested parallel regions

Description:Specifies the initial value for the maximum number of nested parallel regions.The value of this variable shall be a positive integer. If undefined, the numberof active levels is unlimited.

See also: Section 2.26 [omp set max active levels], page 13


3.6 OMP_MAX_TASK_PRIORITY – Set the maximum priority

number that can be set for a task.

Description:Specifies the initial value for the maximum priority value that can be set fora task. The value of this variable shall be a non-negative integer, and zero isallowed. If undefined, the default priority is 0.

See also: Section 2.8 [omp get max task priority], page 7


3.7 OMP_NESTED – Nested parallel regions

Description:Enable or disable nested parallel regions, i.e., whether team members are al-lowed to create new teams. The value of this environment variable shall beTRUE or FALSE. If undefined, nested parallel regions are disabled by default.

See also: Section 2.27 [omp set nested], page 13


3.8 OMP_NUM_THREADS – Specifies the number of threads to use

Description:Specifies the default number of threads to use in parallel regions. The valueof this variable shall be a comma-separated list of positive integers; the valuespecified the number of threads to use for the corresponding nested level. Ifundefined one thread per CPU is used.

See also: Section 2.28 [omp set num threads], page 14


3.9 OMP_PROC_BIND – Whether theads may be moved betweenCPUs

Description:Specifies whether threads may be moved between processors. If set to TRUE,OpenMP theads should not be moved; if set to FALSE they may be moved. Al-ternatively, a comma separated list with the values MASTER, CLOSE and SPREAD






can be used to specify the thread affinity policy for the corresponding nest-ing level. With MASTER the worker threads are in the same place partition asthe master thread. With CLOSE those are kept close to the master thread incontiguous place partitions. And with SPREAD a sparse distribution across theplace partitions is used.

When undefined, OMP_PROC_BIND defaults to TRUE when OMP_PLACES or GOMP_CPU_AFFINITY is set and FALSE otherwise.

See also: Section 3.10 [OMP PLACES], page 21, Section 3.15 [GOMP CPU AFFINITY],page 23, Section 2.15 [omp get proc bind], page 9


3.10 OMP_PLACES – Specifies on which CPUs the theadsshould be placed

Description:The thread placement can be either specified using an abstract name or by anexplicit list of the places. The abstract names threads, cores and socketscan be optionally followed by a positive number in parentheses, which denotesthe how many places shall be created. With threads each place correspondsto a single hardware thread; cores to a single core with the correspondingnumber of hardware threads; and with sockets the place corresponds to a singlesocket. The resulting placement can be shown by setting the OMP_DISPLAY_ENVenvironment variable.

Alternatively, the placement can be specified explicitly as comma-separated listof places. A place is specified by set of nonnegative numbers in curly braces,denoting the denoting the hardware threads. The hardware threads belongingto a place can either be specified as comma-separated list of nonnegative threadnumbers or using an interval. Multiple places can also be either specified bya comma-separated list of places or by an interval. To specify an interval, acolon followed by the count is placed after after the hardware thread numberor the place. Optionally, the length can be followed by a colon and the stridenumber – otherwise a unit stride is assumed. For instance, the following specifiesthe same places list: "{0,1,2}, {3,4,6}, {7,8,9}, {10,11,12}"; "{0:3},{3:3}, {7:3}, {10:3}"; and "{0:2}:4:3".

If OMP_PLACES and GOMP_CPU_AFFINITY are unset and OMP_PROC_BIND is eitherunset or false, threads may be moved between CPUs following no placementpolicy.

See also: Section 3.9 [OMP PROC BIND], page 20, Section 3.15 [GOMP CPU AFFINITY],page 23, Section 2.15 [omp get proc bind], page 9, Section 3.2[OMP DISPLAY ENV], page 19




22 GNU libgomp

3.11 OMP_STACKSIZE – Set default thread stack size

Description:Set the default thread stack size in kilobytes, unless the number is suffixed by B,K, M or G, in which case the size is, respectively, in bytes, kilobytes, megabytes orgigabytes. This is different from pthread_attr_setstacksize which gets thenumber of bytes as an argument. If the stack size cannot be set due to systemconstraints, an error is reported and the initial stack size is left unchanged. Ifundefined, the stack size is system dependent.


3.12 OMP_SCHEDULE – How threads are scheduled

Description:Allows to specify schedule type and chunk size. The value of the variableshall have the form: type[,chunk] where type is one of static, dynamic,guided or auto The optional chunk size shall be a positive integer. If undefined,dynamic scheduling and a chunk size of 1 is used.

See also: Section 2.29 [omp set schedule], page 14

Reference: OpenMP specification v4.5, Sections 2.7.1.1 and 4.1

3.13 OMP_THREAD_LIMIT – Set the maximum number ofthreads

Description:Specifies the number of threads to use for the whole program. The value of thisvariable shall be a positive integer. If undefined, the number of threads is notlimited.

See also: Section 3.8 [OMP NUM THREADS], page 20, Section 2.19[omp get thread limit], page 11


3.14 OMP_WAIT_POLICY – How waiting threads are handled

Description:Specifies whether waiting threads should be active or passive. If the valueis PASSIVE, waiting threads should not consume CPU power while waiting;while the value is ACTIVE specifies that they should. If undefined, threads waitactively for a short time before waiting passively.

See also: Section 3.18 [GOMP SPINCOUNT], page 24







3.15 GOMP_CPU_AFFINITY – Bind threads to specific CPUs

Description:Binds threads to specific CPUs. The variable should contain a space-separatedor comma-separated list of CPUs. This list may contain different kinds ofentries: either single CPU numbers in any order, a range of CPUs (M-N) or arange with some stride (M-N:S). CPU numbers are zero based. For example,GOMP_CPU_AFFINITY="0 3 1-2 4-15:2" will bind the initial thread to CPU 0,the second to CPU 3, the third to CPU 1, the fourth to CPU 2, the fifth toCPU 4, the sixth through tenth to CPUs 6, 8, 10, 12, and 14 respectively andthen start assigning back from the beginning of the list. GOMP_CPU_AFFINITY=0binds all threads to CPU 0.

There is no libgomp library routine to determine whether a CPU affinity spec-ification is in effect. As a workaround, language-specific library functions, e.g.,getenv in C or GET_ENVIRONMENT_VARIABLE in Fortran, may be used to querythe setting of the GOMP_CPU_AFFINITY environment variable. A defined CPUaffinity on startup cannot be changed or disabled during the runtime of theapplication.

If both GOMP_CPU_AFFINITY and OMP_PROC_BIND are set, OMP_PROC_BIND has ahigher precedence. If neither has been set and OMP_PROC_BIND is unset, or whenOMP_PROC_BIND is set to FALSE, the host system will handle the assignment ofthreads to CPUs.

See also: Section 3.10 [OMP PLACES], page 21, Section 3.9 [OMP PROC BIND],page 20

3.16 GOMP_DEBUG – Enable debugging output

Description:Enable debugging output. The variable should be set to 0 (disabled, also thedefault if not set), or 1 (enabled).

If enabled, some debugging output will be printed during execution. This iscurrently not specified in more detail, and subject to change.

3.17 GOMP_STACKSIZE – Set default thread stack size

Description:Set the default thread stack size in kilobytes. This is different from pthread_attr_setstacksize which gets the number of bytes as an argument. If thestack size cannot be set due to system constraints, an error is reported andthe initial stack size is left unchanged. If undefined, the stack size is systemdependent.

See also: Section 3.11 [OMP STACKSIZE], page 22

Reference: GCC Patches Mailinglist, GCC Patches Mailinglist

http://gcc.gnu.org/ml/gcc-patches/2006-06/msg00493.html

http://gcc.gnu.org/ml/gcc-patches/2006-06/msg00496.html

24 GNU libgomp

3.18 GOMP_SPINCOUNT – Set the busy-wait spin count

Description:Determines how long a threads waits actively with consuming CPU power be-fore waiting passively without consuming CPU power. The value may be eitherINFINITE, INFINITY to always wait actively or an integer which gives the num-ber of spins of the busy-wait loop. The integer may optionally be followedby the following suffixes acting as multiplication factors: k (kilo, thousand), M(mega, million), G (giga, billion), or T (tera, trillion). If undefined, 0 is usedwhen OMP_WAIT_POLICY is PASSIVE, 300,000 is used when OMP_WAIT_POLICY isundefined and 30 billion is used when OMP_WAIT_POLICY is ACTIVE. If there aremore OpenMP threads than available CPUs, 1000 and 100 spins are used forOMP_WAIT_POLICY being ACTIVE or undefined, respectively; unless the GOMP_SPINCOUNT is lower or OMP_WAIT_POLICY is PASSIVE.

See also: Section 3.14 [OMP WAIT POLICY], page 22

3.19 GOMP_RTEMS_THREAD_POOLS – Set the RTEMS specificthread pools

Description:This environment variable is only used on the RTEMS real-time operatingsystem. It determines the scheduler instance specific thread pools. Theformat for GOMP_RTEMS_THREAD_POOLS is a list of optional <thread-pool-count>[$<priority>]@<scheduler-name> configurations separated by :where:• <thread-pool-count> is the thread pool count for this scheduler instance.• $<priority> is an optional priority for the worker threads of a thread

pool according to pthread_setschedparam. In case a priority value isomitted, then a worker thread will inherit the priority of the OpenMPmaster thread that created it. The priority of the worker thread is notchanged after creation, even if a new OpenMP master thread using theworker has a different priority.

• @<scheduler-name> is the scheduler instance name according to theRTEMS application configuration.

In case no thread pool configuration is specified for a scheduler instance, theneach OpenMP master thread of this scheduler instance will use its own dynam-ically allocated thread pool. To limit the worker thread count of the threadpools, each OpenMP master thread must call omp_set_num_threads.

Example: Lets suppose we have three scheduler instances IO, WRK0, and WRK1 with GOMP_RTEMS_THREAD_POOLS set to "1@WRK0:3$4@WRK1". Then there are no threadpool restrictions for scheduler instance IO. In the scheduler instance WRK0 thereis one thread pool available. Since no priority is specified for this schedulerinstance, the worker thread inherits the priority of the OpenMP master threadthat created it. In the scheduler instance WRK1 there are three thread poolsavailable and their worker threads run at priority four.

Chapter 4: Enabling OpenACC 25

4 Enabling OpenACC

To activate the OpenACC extensions for C/C++ and Fortran, the compile-time flag‘-fopenacc’ must be specified. This enables the OpenACC directive #pragma acc inC/C++ and !$accp directives in free form, c$acc, *$acc and !$acc directives in fixedform, !$ conditional compilation sentinels in free form and c$, *$ and !$ sentinels in fixedform, for Fortran. The flag also arranges for automatic linking of the OpenACC runtimelibrary (Chapter 5 [OpenACC Runtime Library Routines], page 27).

A complete description of all OpenACC directives accepted may be found in theOpenACC Application Programming Interface manual, version 2.0.

Note that this is an experimental feature and subject to change in future versions ofGCC. See https://gcc.gnu.org/wiki/OpenACC for more information.


https://gcc.gnu.org/wiki/OpenACC

Chapter 5: OpenACC Runtime Library Routines 27

5 OpenACC Runtime Library Routines

The runtime routines described here are defined by section 3 of the OpenACC specificationsin version 2.0. They have C linkage, and do not throw exceptions. Generally, they areavailable only for the host, with the exception of acc_on_device, which is available forboth the host and the acceleration device.

5.1 acc_get_num_devices – Get number of devices for givendevice type

DescriptionThis function returns a value indicating the number of devices available for thedevice type specified in devicetype.

C/C++:Prototype: int acc_get_num_devices(acc_device_t devicetype);

Fortran:Interface: integer function acc_get_num_devices(devicetype)

integer(kind=acc_device_kind) devicetype

Reference: OpenACC specification v2.0, section 3.2.1.

5.2 acc_set_device_type – Set type of device accelerator touse.

DescriptionThis function indicates to the runtime library which device typr, specified indevicetype, to use when executing a parallel or kernels region.

C/C++:Prototype: acc_set_device_type(acc_device_t devicetype);

Fortran:Interface: subroutine acc_set_device_type(devicetype)

integer(kind=acc_device_kind) devicetype


5.3 acc_get_device_type – Get type of device accelerator tobe used.

DescriptionThis function returns what device type will be used when executing a parallelor kernels region.

C/C++:Prototype: acc_device_t acc_get_device_type(void);

Fortran:Interface: function acc_get_device_type(void)

integer(kind=acc_device_kind) acc_get_device_type



28 GNU libgomp


5.4 acc_set_device_num – Set device number to use.

DescriptionThis function will indicate to the runtime which device number, specified bynum, associated with the specifed device type devicetype.

C/C++:Prototype: acc_set_device_num(int num, acc_device_t devicetype);

Fortran:Interface: subroutine acc_set_device_num(devicenum, devicetype)

integer devicenuminteger(kind=acc_device_kind) devicetype


5.5 acc_get_device_num – Get device number to be used.

DescriptionThis function returns which device number associated with the specified devicetype devicetype, will be used when executing a parallel or kernels region.

C/C++:Prototype: int acc_get_device_num(acc_device_t devicetype);

Fortran:Interface: function acc_get_device_num(devicetype)

integer(kind=acc_device_kind) devicetypeinteger acc_get_device_num


5.6 acc_async_test – Test for completion of a specificasynchronous operation.

DescriptionThis function tests for completion of the asynchrounous operation specifiedin arg. In C/C++, a non-zero value will be returned to indicate the specifiedasynchronous operation has completed. While Fortran will return a true. If theasynchrounous operation has not completed, C/C++ returns a zero and Fortranreturns a false.

C/C++:Prototype: int acc_async_test(int arg);

Fortran:Interface: function acc_async_test(arg)

integer(kind=acc_handle_kind) arglogical acc_async_test







5.7 acc_async_test_all – Tests for completion of allasynchronous operations.

DescriptionThis function tests for completion of all asynchrounous operations. In C/C++,a non-zero value will be returned to indicate all asynchronous operations havecompleted. While Fortran will return a true. If any asynchronous operationhas not completed, C/C++ returns a zero and Fortran returns a false.

C/C++:

Prototype: int acc_async_test_all(void);

Fortran:

Interface: function acc_async_test()logical acc_get_device_num


5.8 acc_wait – Wait for completion of a specificasynchronous operation.

DescriptionThis function waits for completion of the asynchronous operation specified inarg.

C/C++:

Prototype: acc_wait(arg);

Fortran:

Interface: subroutine acc_wait(arg)integer(acc_handle_kind) arg


5.9 acc_wait_all – Waits for completion of all asynchronousoperations.

DescriptionThis function waits for the completion of all asynchronous operations.

C/C++:

Prototype: acc_wait_all(void);

Fortran:

Interface: subroutine acc_wait_async()





30 GNU libgomp

5.10 acc_wait_all_async – Wait for completion of allasynchronous operations.

DescriptionThis function enqueues a wait operation on the queue async for any and allasynchronous operations that have been previously enqueued on any queue.

C/C++:Prototype: acc_wait_all_async(int async);

Fortran:Interface: subroutine acc_wait_all_async(async)

integer(acc_handle_kind) async


5.11 acc_wait_async – Wait for completion of asynchronousoperations.

DescriptionThis function enqueues a wait operation on queue async for any and all asyn-chronous operations enqueued on queue arg.

C/C++:Prototype: acc_wait_async(int arg, int async);

Fortran:Interface: subroutine acc_wait_async(arg, async)

integer(acc_handle_kind) arg, async


5.12 acc_init – Initialize runtime for a specific device type.

DescriptionThis function initializes the runtime for the device type specified in devicetype.

C/C++:Prototype: acc_init(acc_device_t devicetype);

Fortran:Interface: subroutine acc_init(devicetype)

integer(acc_device_kind) devicetype


5.13 acc_shutdown – Shuts down the runtime for a specificdevice type.

DescriptionThis function shuts down the runtime for the device type specified in devicetype.

C/C++:Prototype: acc_shutdown(acc_device_t devicetype);





Fortran:

Interface: subroutine acc_shutdown(devicetype)integer(acc_device_kind) devicetype


5.14 acc_on_device – Whether executing on a particulardevice

Description:This function returns whether the program is executing on a particular devicespecified in devicetype. In C/C++ a non-zero value is returned to indicatethe device is execiting on the specified device type. In Fortran, true will bereturned. If the program is not executing on the specified device type C/C++will return a zero, while Fortran will return false.

C/C++:

Prototype: acc_on_device(acc_device_t devicetype);

Fortran:

Interface: function acc_on_device(devicetype)integer(acc_device_kind) devicetypelogical acc_on_device


5.15 acc_malloc – Allocate device memory.

DescriptionThis function allocates len bytes of device memory. It returns the device addressof the allocated memory.

C/C++:

Prototype: d_void* acc_malloc(size_t len);


5.16 acc_free – Free device memory.

DescriptionFree previously allocated device memory at the device address a.

C/C++:

Prototype: acc_free(d_void *a);






32 GNU libgomp

5.17 acc_copyin – Allocate device memory and copy hostmemory to it.

DescriptionIn C/C++, this function allocates len bytes of device memory and maps it to thespecified host address in a. The device address of the newly allocated devicememory is returned.In Fortran, two (2) forms are supported. In the first form, a specifies a con-tiguous array section. The second form a specifies a variable or array elementand len specifies the length in bytes.

C/C++:Prototype: void *acc_copyin(h_void *a, size_t len);

Fortran:Interface: subroutine acc_copyin(a)

type, dimension(:[,:]...) :: aInterface: subroutine acc_copyin(a, len)

type, dimension(:[,:]...) :: ainteger len


5.18 acc_present_or_copyin – If the data is not present onthe device, allocate device memory and copy from hostmemory.

DescriptionThis function tests if the host data specifed by a and of length len is presentor not. If it is not present, then device memory will be allocated and the hostmemory copied. The device address of the newly allocated device memory isreturned.In Fortran, two (2) forms are supported. In the first form, a specifies a con-tiguous array section. The second form a specifies a variable or array elementand len specifies the length in bytes.

C/C++:Prototype: void *acc_present_or_copyin(h_void *a, size_t len);Prototype: void *acc_pcopyin(h_void *a, size_t len);

Fortran:Interface: subroutine acc_present_or_copyin(a)

type, dimension(:[,:]...) :: aInterface: subroutine acc_present_or_copyin(a, len)


Interface: subroutine acc_pcopyin(a)type, dimension(:[,:]...) :: a

Interface: subroutine acc_pcopyin(a, len)type, dimension(:[,:]...) :: a



integer len


5.19 acc_create – Allocate device memory and map it tohost memory.

DescriptionThis function allocates device memory and maps it to host memory specified bythe host address a with a length of len bytes. In C/C++, the function returnsthe device address of the allocated device memory.In Fortran, two (2) forms are supported. In the first form, a specifies a con-tiguous array section. The second form a specifies a variable or array elementand len specifies the length in bytes.

C/C++:Prototype: void *acc_create(h_void *a, size_t len);

Fortran:Interface: subroutine acc_create(a)

type, dimension(:[,:]...) :: aInterface: subroutine acc_create(a, len)



5.20 acc_present_or_create – If the data is not present onthe device, allocate device memory and map it to hostmemory.

DescriptionThis function tests if the host data specifed by a and of length len is presentor not. If it is not present, then device memory will be allocated and mappedto host memory. In C/C++, the device address of the newly allocated devicememory is returned.In Fortran, two (2) forms are supported. In the first form, a specifies a con-tiguous array section. The second form a specifies a variable or array elementand len specifies the length in bytes.

C/C++:Prototype: void *acc_present_or_create(h_void *a, size_t len)Prototype: void *acc_pcreate(h_void *a, size_t len)

Fortran:Interface: subroutine acc_present_or_create(a)

type, dimension(:[,:]...) :: aInterface: subroutine acc_present_or_create(a, len)

type, dimension(:[,:]...) :: a



34 GNU libgomp

integer lenInterface: subroutine acc_pcreate(a)

type, dimension(:[,:]...) :: aInterface: subroutine acc_pcreate(a, len)



5.21 acc_copyout – Copy device memory to host memory.

DescriptionThis function copies mapped device memory to host memory which is specifiedby host address a for a length len bytes in C/C++.In Fortran, two (2) forms are supported. In the first form, a specifies a con-tiguous array section. The second form a specifies a variable or array elementand len specifies the length in bytes.

C/C++:Prototype: acc_copyout(h_void *a, size_t len);

Fortran:Interface: subroutine acc_copyout(a)

type, dimension(:[,:]...) :: aInterface: subroutine acc_copyout(a, len)



5.22 acc_delete – Free device memory.

DescriptionThis function frees previously allocated device memory specified by the deviceaddress a and the length of len bytes.In Fortran, two (2) forms are supported. In the first form, a specifies a con-tiguous array section. The second form a specifies a variable or array elementand len specifies the length in bytes.

C/C++:Prototype: acc_delete(h_void *a, size_t len);

Fortran:Interface: subroutine acc_delete(a)

type, dimension(:[,:]...) :: aInterface: subroutine acc_delete(a, len)







5.23 acc_update_device – Update device memory frommapped host memory.

DescriptionThis function updates the device copy from the previously mapped host mem-ory. The host memory is specified with the host address a and a length of lenbytes.

In Fortran, two (2) forms are supported. In the first form, a specifies a con-tiguous array section. The second form a specifies a variable or array elementand len specifies the length in bytes.

C/C++:

Prototype: acc_update_device(h_void *a, size_t len);

Fortran:

Interface: subroutine acc_update_device(a)type, dimension(:[,:]...) :: a

Interface: subroutine acc_update_device(a, len)type, dimension(:[,:]...) :: ainteger len


5.24 acc_update_self – Update host memory from mappeddevice memory.

DescriptionThis function updates the host copy from the previously mapped device mem-ory. The host memory is specified with the host address a and a length of lenbytes.

In Fortran, two (2) forms are supported. In the first form, a specifies a con-tiguous array section. The second form a specifies a variable or array elementand len specifies the length in bytes.

C/C++:

Prototype: acc_update_self(h_void *a, size_t len);

Fortran:

Interface: subroutine acc_update_self(a)type, dimension(:[,:]...) :: a

Interface: subroutine acc_update_self(a, len)type, dimension(:[,:]...) :: ainteger len




36 GNU libgomp

5.25 acc_map_data – Map previously allocated devicememory to host memory.

DescriptionThis function maps previously allocated device and host memory. The devicememory is specified with the device address d. The host memory is specifiedwith the host address h and a length of len.

C/C++:

Prototype: acc_map_data(h_void *h, d_void *d, size_t len);


5.26 acc_unmap_data – Unmap device memory from hostmemory.

DescriptionThis function unmaps previously mapped device and host memory. The latterspecified by h.

C/C++:

Prototype: acc_unmap_data(h_void *h);


5.27 acc_deviceptr – Get device pointer associated withspecific host address.

DescriptionThis function returns the device address that has been mapped to the hostaddress specified by h.

C/C++:

Prototype: void *acc_deviceptr(h_void *h);


5.28 acc_hostptr – Get host pointer associated with specificdevice address.

DescriptionThis function returns the host address that has been mapped to the deviceaddress specified by d.

C/C++:

Prototype: void *acc_hostptr(d_void *d);







5.29 acc_is_present – Indicate whether host variable / arrayis present on device.

DescriptionThis function indicates whether the specified host address in a and a length oflen bytes is present on the device. In C/C++, a non-zero value is returned toindicate the presence of the mapped memory on the device. A zero is returnedto indicate the memory is not mapped on the device.In Fortran, two (2) forms are supported. In the first form, a specifies a con-tiguous array section. The second form a specifies a variable or array elementand len specifies the length in bytes. If the host memory is mapped to devicememory, then a true is returned. Otherwise, a false is return to indicate themapped memory is not present.

C/C++:Prototype: int acc_is_present(h_void *a, size_t len);

Fortran:Interface: function acc_is_present(a)

type, dimension(:[,:]...) :: alogical acc_is_present

Interface: function acc_is_present(a, len)type, dimension(:[,:]...) :: ainteger lenlogical acc_is_present


5.30 acc_memcpy_to_device – Copy host memory to devicememory.

DescriptionThis function copies host memory specified by host address of src to devicememory specified by the device address dest for a length of bytes bytes.

C/C++:Prototype: acc_memcpy_to_device(d_void *dest, h_void *src, size_t

bytes);


5.31 acc_memcpy_from_device – Copy device memory to hostmemory.

DescriptionThis function copies host memory specified by host address of src from devicememory specified by the device address dest for a length of bytes bytes.

C/C++:Prototype: acc_memcpy_from_device(d_void *dest, h_void *src, size_t

bytes);





38 GNU libgomp

5.32 acc_get_current_cuda_device – Get CUDA devicehandle.

DescriptionThis function returns the CUDA device handle. This handle is the same asused by the CUDA Runtime or Driver API’s.

C/C++:Prototype: void *acc_get_current_cuda_device(void);

Reference: OpenACC specification v2.0, section A.2.1.1.

5.33 acc_get_current_cuda_context – Get CUDA contexthandle.

DescriptionThis function returns the CUDA context handle. This handle is the same asused by the CUDA Runtime or Driver API’s.

C/C++:Prototype: acc_get_current_cuda_context(void);


5.34 acc_get_cuda_stream – Get CUDA stream handle.

DescriptionThis function returns the CUDA stream handle. This handle is the same asused by the CUDA Runtime or Driver API’s.

C/C++:Prototype: acc_get_cuda_stream(void);


5.35 acc_set_cuda_stream – Set CUDA stream handle.

DescriptionThis function associates the stream handle specified by stream with the asyn-chronous value specified by async.

C/C++:Prototype: acc_set_cuda_stream(int async void *stream);






Chapter 6: OpenACC Environment Variables 39

6 OpenACC Environment Variables

The variables ACC_DEVICE_TYPE and ACC_DEVICE_NUM are defined by section 4 of the Ope-nACC specification in version 2.0. The variable GCC_ACC_NOTIFY is used for diagnosticpurposes.

6.1 ACC_DEVICE_TYPE

Reference: OpenACC specification v2.0, section 4.1.

6.2 ACC_DEVICE_NUM

Reference: OpenACC specification v2.0, section 4.2.

6.3 GCC_ACC_NOTIFY

Description:Print debug information pertaining to the accelerator.



Chapter 7: CUDA Streams Usage 41

7 CUDA Streams Usage

This applies to the nvptx plugin only.The library provides elements that perform asynchronous movement of data and asyn-

chronous operation of computing constructs. This asynchronous functionality is imple-mented by making use of CUDA streams1.

The primary means by that the asychronous functionality is accessed is through theuse of those OpenACC directives which make use of the async and wait clauses. Whenthe async clause is first used with a directive, it creates a CUDA stream. If an async-argument is used with the async clause, then the stream is associated with the specifiedasync-argument.

Following the creation of an association between a CUDA stream and the async-argument of an async clause, both the wait clause and the wait directive can be used.When either the clause or directive is used after stream creation, it creates a rendezvouspoint whereby execution waits until all operations associated with the async-argument,that is, stream, have completed.

Normally, the management of the streams that are created as a result of using the asyncclause, is done without any intervention by the caller. This implies the association betweenthe async-argument and the CUDA stream will be maintained for the lifetime of theprogram. However, this association can be changed through the use of the library functionacc_set_cuda_stream. When the function acc_set_cuda_stream is called, the CUDAstream that was originally associated with the async clause will be destroyed. Cautionshould be taken when changing the association as subsequent references to the async-argument refer to a different CUDA stream.

1 See "Stream Management" in "CUDA Driver API", TRM-06703-001, Version 5.5, for additionalinformation

Chapter 8: OpenACC Library Interoperability 43

8 OpenACC Library Interoperability

8.1 Introduction

The OpenACC library uses the CUDA Driver API, and may interact with programs thatuse the Runtime library directly, or another library based on the Runtime library, e.g.,CUBLAS1. This chapter describes the use cases and what changes are required in order touse both the OpenACC library and the CUBLAS and Runtime libraries within a program.

8.2 First invocation: NVIDIA CUBLAS library API

In this first use case (see below), a function in the CUBLAS library is called prior to anyof the functions in the OpenACC library. More specifically, the function cublasCreate().

When invoked, the function initializes the library and allocates the hardware resourceson the host and the device on behalf of the caller. Once the initialization and allocationhas completed, a handle is returned to the caller. The OpenACC library also requiresinitialization and allocation of hardware resources. Since the CUBLAS library has alreadyallocated the hardware resources for the device, all that is left to do is to initialize theOpenACC library and acquire the hardware resources on the host.

Prior to calling the OpenACC function that initializes the library and allocate the hosthardware resources, you need to acquire the device number that was allocated during thecall to cublasCreate(). The invoking of the runtime library function cudaGetDevice()accomplishes this. Once acquired, the device number is passed along with the device typeas parameters to the OpenACC library function acc_set_device_num().

Once the call to acc_set_device_num() has completed, the OpenACC library uses thecontext that was created during the call to cublasCreate(). In other words, both librarieswill be sharing the same context.

/* Create the handle */

s = cublasCreate(&h);

if (s != CUBLAS_STATUS_SUCCESS)

{

fprintf(stderr, "cublasCreate failed %d\n", s);

exit(EXIT_FAILURE);

}

/* Get the device number */

e = cudaGetDevice(&dev);

if (e != cudaSuccess)

{

fprintf(stderr, "cudaGetDevice failed %d\n", e);

exit(EXIT_FAILURE);

}

/* Initialize OpenACC library and use device ’dev’ */

acc_set_device_num(dev, acc_device_nvidia);

Use Case 11 See section 2.26, "Interactions with the CUDA Driver API" in "CUDA Runtime API", Version 5.5,

and section 2.27, "VDPAU Interoperability", in "CUDA Driver API", TRM-06703-001, Version 5.5, foradditional information on library interoperability.

44 GNU libgomp

8.3 First invocation: OpenACC library API

In this second use case (see below), a function in the OpenACC library is called prior to anyof the functions in the CUBLAS library. More specificially, the function acc_set_device_num().

In the use case presented here, the function acc_set_device_num() is used to bothinitialize the OpenACC library and allocate the hardware resources on the host and thedevice. In the call to the function, the call parameters specify which device to use and whatdevice type to use, i.e., acc_device_nvidia. It should be noted that this is but one methodto initialize the OpenACC library and allocate the appropriate hardware resources. Othermethods are available through the use of environment variables and these will be discussedin the next section.

Once the call to acc_set_device_num() has completed, other OpenACC functions canbe called as seen with multiple calls being made to acc_copyin(). In addition, calls canbe made to functions in the CUBLAS library. In the use case a call to cublasCreate() ismade subsequent to the calls to acc_copyin(). As seen in the previous use case, a call tocublasCreate() initializes the CUBLAS library and allocates the hardware resources on thehost and the device. However, since the device has already been allocated, cublasCreate()will only initialize the CUBLAS library and allocate the appropriate hardware resources onthe host. The context that was created as part of the OpenACC initialization is sharedwith the CUBLAS library, similarly to the first use case.

dev = 0;

acc_set_device_num(dev, acc_device_nvidia);

/* Copy the first set to the device */

d_X = acc_copyin(&h_X[0], N * sizeof (float));

if (d_X == NULL)

{

fprintf(stderr, "copyin error h_X\n");

exit(EXIT_FAILURE);

}

/* Copy the second set to the device */

d_Y = acc_copyin(&h_Y1[0], N * sizeof (float));

if (d_Y == NULL)

{

fprintf(stderr, "copyin error h_Y1\n");

exit(EXIT_FAILURE);

}

/* Create the handle */

s = cublasCreate(&h);


{

fprintf(stderr, "cublasCreate failed %d\n", s);

exit(EXIT_FAILURE);

}

/* Perform saxpy using CUBLAS library function */

s = cublasSaxpy(h, N, &alpha, d_X, 1, d_Y, 1);


{

Chapter 8: OpenACC Library Interoperability 45

fprintf(stderr, "cublasSaxpy failed %d\n", s);

exit(EXIT_FAILURE);

}

/* Copy the results from the device */

acc_memcpy_from_device(&h_Y1[0], d_Y, N * sizeof (float));

Use Case 2

8.4 OpenACC library and environment variables

There are two environment variables associated with the OpenACC library that may beused to control the device type and device number: ACC_DEVICE_TYPE and ACC_DEVICE_NUM, respecively. These two environement variables can be used as an alternative to callingacc_set_device_num(). As seen in the second use case, the device type and device numberwere specified using acc_set_device_num(). If however, the aforementioned environmentvariables were set, then the call to acc_set_device_num() would not be required.

The use of the environment variables is only relevant when an OpenACC function iscalled prior to a call to cudaCreate(). If cudaCreate() is called prior to a call to anOpenACC function, then you must call acc_set_device_num()2

2 More complete information about ACC_DEVICE_TYPE and ACC_DEVICE_NUM can be found in sections 4.1and 4.2 of the OpenACC Application Programming Interface, Version 2.0.


Chapter 9: The libgomp ABI 47

9 The libgomp ABI

The following sections present notes on the external ABI as presented by libgomp. Onlymaintainers should need them.

9.1 Implementing MASTER constructif (omp_get_thread_num () == 0)

block

Alternately, we generate two copies of the parallel subfunction and only include this inthe version run by the master thread. Surely this is not worthwhile though...

9.2 Implementing CRITICAL construct

Without a specified name,void GOMP_critical_start (void);

void GOMP_critical_end (void);

so that we don’t get COPY relocations from libgomp to the main application.

With a specified name, use omp set lock and omp unset lock with name being trans-formed into a variable declared like

omp_lock_t gomp_critical_user_<name> __attribute__((common))

Ideally the ABI would specify that all zero is a valid unlocked state, and so we wouldn’tneed to initialize this at startup.

9.3 Implementing ATOMIC construct

The target should implement the __sync builtins.

Failing that we could addvoid GOMP_atomic_enter (void)

void GOMP_atomic_exit (void)

which reuses the regular lock code, but with yet another lock object private to the library.

9.4 Implementing FLUSH construct

Expands to the __sync_synchronize builtin.

9.5 Implementing BARRIER constructvoid GOMP_barrier (void)

9.6 Implementing THREADPRIVATE construct

In most cases we can map this directly to __thread. Except that OMP allows constructorsfor C++ objects. We can either refuse to support this (how often is it used?) or we canimplement something akin to .ctors.

Even more ideally, this ctor feature is handled by extensions to the main pthreads library.Failing that, we can have a set of entry points to register ctor functions to be called.

48 GNU libgomp

9.7 Implementing PRIVATE clause

In association with a PARALLEL, or within the lexical extent of a PARALLEL block, thevariable becomes a local variable in the parallel subfunction.

In association with FOR or SECTIONS blocks, create a new automatic variable withinthe current function. This preserves the semantic of new variable creation.

9.8 Implementing FIRSTPRIVATE LASTPRIVATECOPYIN and COPYPRIVATE clauses

This seems simple enough for PARALLEL blocks. Create a private struct for communicatingbetween the parent and subfunction. In the parent, copy in values for scalar and "small"structs; copy in addresses for others TREE ADDRESSABLE types. In the subfunction,copy the value into the local variable.

It is not clear what to do with bare FOR or SECTION blocks. The only thing I canfigure is that we do something like:

#pragma omp for firstprivate(x) lastprivate(y)

for (int i = 0; i < n; ++i)

body;

which becomes{

int x = x, y;

// for stuff

if (i == n)

y = y;

}

where the "x=x" and "y=y" assignments actually have different uids for the two vari-ables, i.e. not something you could write directly in C. Presumably this only makes senseif the "outer" x and y are global variables.

COPYPRIVATE would work the same way, except the structure broadcast would haveto happen via SINGLE machinery instead.

9.9 Implementing REDUCTION clause

The private struct mentioned in the previous section should have a pointer to an array ofthe type of the variable, indexed by the thread’s team id. The thread stores its final valueinto the array, and after the barrier, the master thread iterates over the array to collect thevalues.

9.10 Implementing PARALLEL construct#pragma omp parallel

{

body;

}

becomesvoid subfunction (void *data)

{


use data;

body;

}

setup data;

GOMP_parallel_start (subfunction, &data, num_threads);

subfunction (&data);

GOMP_parallel_end ();

void GOMP_parallel_start (void (*fn)(void *), void *data, unsigned num_threads)

The FN argument is the subfunction to be run in parallel.The DATA argument is a pointer to a structure used to communicate data in and out

of the subfunction, as discussed above with respect to FIRSTPRIVATE et al.The NUM THREADS argument is 1 if an IF clause is present and false, or the value of

the NUM THREADS clause, if present, or 0.The function needs to create the appropriate number of threads and/or launch them

from the dock. It needs to create the team structure and assign team ids.void GOMP_parallel_end (void)

Tears down the team and returns us to the previous omp_in_parallel() state.

9.11 Implementing FOR construct#pragma omp parallel for

for (i = lb; i <= ub; i++)

body;

becomesvoid subfunction (void *data)

{

long _s0, _e0;

while (GOMP_loop_static_next (&_s0, &_e0))

{

long _e1 = _e0, i;

for (i = _s0; i < _e1; i++)

body;

}

GOMP_loop_end_nowait ();

}

GOMP_parallel_loop_static (subfunction, NULL, 0, lb, ub+1, 1, 0);

subfunction (NULL);

GOMP_parallel_end ();

#pragma omp for schedule(runtime)

for (i = 0; i < n; i++)

body;

becomes{

long i, _s0, _e0;

if (GOMP_loop_runtime_start (0, n, 1, &_s0, &_e0))

do {

long _e1 = _e0;

for (i = _s0, i < _e0; i++)

body;

} while (GOMP_loop_runtime_next (&_s0, _&e0));

50 GNU libgomp

GOMP_loop_end ();

}

Note that while it looks like there is trickiness to propagating a non-constant STEP,there isn’t really. We’re explicitly allowed to evaluate it as many times as we want, andany variables involved should automatically be handled as PRIVATE or SHARED like anyother variables. So the expression should remain evaluable in the subfunction. We can alsopull it into a local variable if we like, but since its supposed to remain unchanged, we canalso not if we like.

If we have SCHEDULE(STATIC), and no ORDERED, then we ought to be able toget away with no work-sharing context at all, since we can simply perform the arithmeticdirectly in each thread to divide up the iterations. Which would mean that we wouldn’tneed to call any of these routines.

There are separate routines for handling loops with an ORDERED clause. Bookkeepingfor that is non-trivial...

9.12 Implementing ORDERED constructvoid GOMP_ordered_start (void)

void GOMP_ordered_end (void)

9.13 Implementing SECTIONS construct

A block as#pragma omp sections

{

#pragma omp section

stmt1;

#pragma omp section

stmt2;

#pragma omp section

stmt3;

}

becomesfor (i = GOMP_sections_start (3); i != 0; i = GOMP_sections_next ())

switch (i)

{

case 1:

stmt1;

break;

case 2:

stmt2;

break;

case 3:

stmt3;

break;

}

GOMP_barrier ();

9.14 Implementing SINGLE construct

A block like


#pragma omp single

{

body;

}

becomesif (GOMP_single_start ())

body;

GOMP_barrier ();

while#pragma omp single copyprivate(x)

body;

becomesdatap = GOMP_single_copy_start ();

if (datap == NULL)

{

body;

data.x = x;

GOMP_single_copy_end (&data);

}

else

x = datap->x;

GOMP_barrier ();

9.15 Implementing OpenACC’s PARALLEL constructvoid GOACC_parallel ()

Chapter 10: Reporting Bugs 53

10 Reporting Bugs

Bugs in the GNU Offloading and Multi Processing Runtime Library should be reported viaBugzilla. Please add "openacc", or "openmp", or both to the keywords field in the bugreport, as appropriate.

http://gcc.gnu.org/bugzilla/

GNU General Public License 55

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How to Apply These Terms to Your New Programs

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This program is free software: you can redistribute it and/or modify

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This program is distributed in the hope that it will be useful, but

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The GNU General Public License does not permit incorporating your program intoproprietary programs. If your program is a subroutine library, you may consider it moreuseful to permit linking proprietary applications with the library. If this is what you wantto do, use the GNU Lesser General Public License instead of this License. But first, pleaseread http://www.gnu.org/philosophy/why-not-lgpl.html.

http://www.gnu.org/licenses/

http://www.gnu.org/licenses/

http://www.gnu.org/philosophy/why-not-lgpl.html

GNU Free Documentation License 67

GNU Free Documentation License

Version 1.3, 3 November 2008Copyright c© 2000, 2001, 2002, 2007, 2008 Free Software Foundation, Inc.http://fsf.org/

Everyone is permitted to copy and distribute verbatim copiesof this license document, but changing it is not allowed.

0. PREAMBLEThe purpose of this License is to make a manual, textbook, or other functional anduseful document free in the sense of freedom: to assure everyone the effective freedomto copy and redistribute it, with or without modifying it, either commercially or non-commercially. Secondarily, this License preserves for the author and publisher a wayto get credit for their work, while not being considered responsible for modificationsmade by others.This License is a kind of “copyleft”, which means that derivative works of the documentmust themselves be free in the same sense. It complements the GNU General PublicLicense, which is a copyleft license designed for free software.We have designed this License in order to use it for manuals for free software, becausefree software needs free documentation: a free program should come with manualsproviding the same freedoms that the software does. But this License is not limited tosoftware manuals; it can be used for any textual work, regardless of subject matter orwhether it is published as a printed book. We recommend this License principally forworks whose purpose is instruction or reference.

1. APPLICABILITY AND DEFINITIONSThis License applies to any manual or other work, in any medium, that contains anotice placed by the copyright holder saying it can be distributed under the termsof this License. Such a notice grants a world-wide, royalty-free license, unlimited induration, to use that work under the conditions stated herein. The “Document”,below, refers to any such manual or work. Any member of the public is a licensee, andis addressed as “you”. You accept the license if you copy, modify or distribute the workin a way requiring permission under copyright law.A “Modified Version” of the Document means any work containing the Document ora portion of it, either copied verbatim, or with modifications and/or translated intoanother language.A “Secondary Section” is a named appendix or a front-matter section of the Documentthat deals exclusively with the relationship of the publishers or authors of the Documentto the Document’s overall subject (or to related matters) and contains nothing thatcould fall directly within that overall subject. (Thus, if the Document is in part atextbook of mathematics, a Secondary Section may not explain any mathematics.) Therelationship could be a matter of historical connection with the subject or with relatedmatters, or of legal, commercial, philosophical, ethical or political position regardingthem.The “Invariant Sections” are certain Secondary Sections whose titles are designated, asbeing those of Invariant Sections, in the notice that says that the Document is released

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68 GNU libgomp

under this License. If a section does not fit the above definition of Secondary then it isnot allowed to be designated as Invariant. The Document may contain zero InvariantSections. If the Document does not identify any Invariant Sections then there are none.

The “Cover Texts” are certain short passages of text that are listed, as Front-CoverTexts or Back-Cover Texts, in the notice that says that the Document is released underthis License. A Front-Cover Text may be at most 5 words, and a Back-Cover Text maybe at most 25 words.

A “Transparent” copy of the Document means a machine-readable copy, representedin a format whose specification is available to the general public, that is suitable forrevising the document straightforwardly with generic text editors or (for images com-posed of pixels) generic paint programs or (for drawings) some widely available drawingeditor, and that is suitable for input to text formatters or for automatic translation toa variety of formats suitable for input to text formatters. A copy made in an otherwiseTransparent file format whose markup, or absence of markup, has been arranged tothwart or discourage subsequent modification by readers is not Transparent. An imageformat is not Transparent if used for any substantial amount of text. A copy that isnot “Transparent” is called “Opaque”.

Examples of suitable formats for Transparent copies include plain ascii withoutmarkup, Texinfo input format, LaTEX input format, SGML or XML using a publiclyavailable DTD, and standard-conforming simple HTML, PostScript or PDF designedfor human modification. Examples of transparent image formats include PNG, XCFand JPG. Opaque formats include proprietary formats that can be read and editedonly by proprietary word processors, SGML or XML for which the DTD and/orprocessing tools are not generally available, and the machine-generated HTML,PostScript or PDF produced by some word processors for output purposes only.

The “Title Page” means, for a printed book, the title page itself, plus such followingpages as are needed to hold, legibly, the material this License requires to appear in thetitle page. For works in formats which do not have any title page as such, “Title Page”means the text near the most prominent appearance of the work’s title, preceding thebeginning of the body of the text.

The “publisher” means any person or entity that distributes copies of the Documentto the public.

A section “Entitled XYZ” means a named subunit of the Document whose title eitheris precisely XYZ or contains XYZ in parentheses following text that translates XYZ inanother language. (Here XYZ stands for a specific section name mentioned below, suchas “Acknowledgements”, “Dedications”, “Endorsements”, or “History”.) To “Preservethe Title” of such a section when you modify the Document means that it remains asection “Entitled XYZ” according to this definition.

The Document may include Warranty Disclaimers next to the notice which states thatthis License applies to the Document. These Warranty Disclaimers are considered tobe included by reference in this License, but only as regards disclaiming warranties:any other implication that these Warranty Disclaimers may have is void and has noeffect on the meaning of this License.

2. VERBATIM COPYING


You may copy and distribute the Document in any medium, either commercially ornoncommercially, provided that this License, the copyright notices, and the licensenotice saying this License applies to the Document are reproduced in all copies, andthat you add no other conditions whatsoever to those of this License. You may not usetechnical measures to obstruct or control the reading or further copying of the copiesyou make or distribute. However, you may accept compensation in exchange for copies.If you distribute a large enough number of copies you must also follow the conditionsin section 3.You may also lend copies, under the same conditions stated above, and you may publiclydisplay copies.

3. COPYING IN QUANTITYIf you publish printed copies (or copies in media that commonly have printed covers) ofthe Document, numbering more than 100, and the Document’s license notice requiresCover Texts, you must enclose the copies in covers that carry, clearly and legibly, allthese Cover Texts: Front-Cover Texts on the front cover, and Back-Cover Texts onthe back cover. Both covers must also clearly and legibly identify you as the publisherof these copies. The front cover must present the full title with all words of the titleequally prominent and visible. You may add other material on the covers in addition.Copying with changes limited to the covers, as long as they preserve the title of theDocument and satisfy these conditions, can be treated as verbatim copying in otherrespects.If the required texts for either cover are too voluminous to fit legibly, you should putthe first ones listed (as many as fit reasonably) on the actual cover, and continue therest onto adjacent pages.If you publish or distribute Opaque copies of the Document numbering more than 100,you must either include a machine-readable Transparent copy along with each Opaquecopy, or state in or with each Opaque copy a computer-network location from whichthe general network-using public has access to download using public-standard networkprotocols a complete Transparent copy of the Document, free of added material. Ifyou use the latter option, you must take reasonably prudent steps, when you begindistribution of Opaque copies in quantity, to ensure that this Transparent copy willremain thus accessible at the stated location until at least one year after the last timeyou distribute an Opaque copy (directly or through your agents or retailers) of thatedition to the public.It is requested, but not required, that you contact the authors of the Document wellbefore redistributing any large number of copies, to give them a chance to provide youwith an updated version of the Document.

4. MODIFICATIONSYou may copy and distribute a Modified Version of the Document under the conditionsof sections 2 and 3 above, provided that you release the Modified Version under preciselythis License, with the Modified Version filling the role of the Document, thus licensingdistribution and modification of the Modified Version to whoever possesses a copy ofit. In addition, you must do these things in the Modified Version:A. Use in the Title Page (and on the covers, if any) a title distinct from that of the

Document, and from those of previous versions (which should, if there were any,

70 GNU libgomp

be listed in the History section of the Document). You may use the same title asa previous version if the original publisher of that version gives permission.

B. List on the Title Page, as authors, one or more persons or entities responsible forauthorship of the modifications in the Modified Version, together with at least fiveof the principal authors of the Document (all of its principal authors, if it has fewerthan five), unless they release you from this requirement.

C. State on the Title page the name of the publisher of the Modified Version, as thepublisher.

D. Preserve all the copyright notices of the Document.E. Add an appropriate copyright notice for your modifications adjacent to the other

copyright notices.F. Include, immediately after the copyright notices, a license notice giving the public

permission to use the Modified Version under the terms of this License, in the formshown in the Addendum below.

G. Preserve in that license notice the full lists of Invariant Sections and required CoverTexts given in the Document’s license notice.

H. Include an unaltered copy of this License.I. Preserve the section Entitled “History”, Preserve its Title, and add to it an item

stating at least the title, year, new authors, and publisher of the Modified Versionas given on the Title Page. If there is no section Entitled “History” in the Docu-ment, create one stating the title, year, authors, and publisher of the Documentas given on its Title Page, then add an item describing the Modified Version asstated in the previous sentence.

J. Preserve the network location, if any, given in the Document for public access toa Transparent copy of the Document, and likewise the network locations given inthe Document for previous versions it was based on. These may be placed in the“History” section. You may omit a network location for a work that was publishedat least four years before the Document itself, or if the original publisher of theversion it refers to gives permission.

K. For any section Entitled “Acknowledgements” or “Dedications”, Preserve the Titleof the section, and preserve in the section all the substance and tone of each of thecontributor acknowledgements and/or dedications given therein.

L. Preserve all the Invariant Sections of the Document, unaltered in their text andin their titles. Section numbers or the equivalent are not considered part of thesection titles.

M. Delete any section Entitled “Endorsements”. Such a section may not be includedin the Modified Version.

N. Do not retitle any existing section to be Entitled “Endorsements” or to conflict intitle with any Invariant Section.

O. Preserve any Warranty Disclaimers.

If the Modified Version includes new front-matter sections or appendices that qualifyas Secondary Sections and contain no material copied from the Document, you may atyour option designate some or all of these sections as invariant. To do this, add their


titles to the list of Invariant Sections in the Modified Version’s license notice. Thesetitles must be distinct from any other section titles.

You may add a section Entitled “Endorsements”, provided it contains nothing butendorsements of your Modified Version by various parties—for example, statements ofpeer review or that the text has been approved by an organization as the authoritativedefinition of a standard.

You may add a passage of up to five words as a Front-Cover Text, and a passage of upto 25 words as a Back-Cover Text, to the end of the list of Cover Texts in the ModifiedVersion. Only one passage of Front-Cover Text and one of Back-Cover Text may beadded by (or through arrangements made by) any one entity. If the Document alreadyincludes a cover text for the same cover, previously added by you or by arrangementmade by the same entity you are acting on behalf of, you may not add another; butyou may replace the old one, on explicit permission from the previous publisher thatadded the old one.

The author(s) and publisher(s) of the Document do not by this License give permissionto use their names for publicity for or to assert or imply endorsement of any ModifiedVersion.

5. COMBINING DOCUMENTS

You may combine the Document with other documents released under this License,under the terms defined in section 4 above for modified versions, provided that youinclude in the combination all of the Invariant Sections of all of the original documents,unmodified, and list them all as Invariant Sections of your combined work in its licensenotice, and that you preserve all their Warranty Disclaimers.

The combined work need only contain one copy of this License, and multiple identicalInvariant Sections may be replaced with a single copy. If there are multiple InvariantSections with the same name but different contents, make the title of each such sectionunique by adding at the end of it, in parentheses, the name of the original author orpublisher of that section if known, or else a unique number. Make the same adjustmentto the section titles in the list of Invariant Sections in the license notice of the combinedwork.

In the combination, you must combine any sections Entitled “History” in the vari-ous original documents, forming one section Entitled “History”; likewise combine anysections Entitled “Acknowledgements”, and any sections Entitled “Dedications”. Youmust delete all sections Entitled “Endorsements.”

6. COLLECTIONS OF DOCUMENTS

You may make a collection consisting of the Document and other documents releasedunder this License, and replace the individual copies of this License in the variousdocuments with a single copy that is included in the collection, provided that youfollow the rules of this License for verbatim copying of each of the documents in allother respects.

You may extract a single document from such a collection, and distribute it individu-ally under this License, provided you insert a copy of this License into the extracteddocument, and follow this License in all other respects regarding verbatim copying ofthat document.

72 GNU libgomp

7. AGGREGATION WITH INDEPENDENT WORKS

A compilation of the Document or its derivatives with other separate and independentdocuments or works, in or on a volume of a storage or distribution medium, is calledan “aggregate” if the copyright resulting from the compilation is not used to limit thelegal rights of the compilation’s users beyond what the individual works permit. Whenthe Document is included in an aggregate, this License does not apply to the otherworks in the aggregate which are not themselves derivative works of the Document.

If the Cover Text requirement of section 3 is applicable to these copies of the Document,then if the Document is less than one half of the entire aggregate, the Document’s CoverTexts may be placed on covers that bracket the Document within the aggregate, or theelectronic equivalent of covers if the Document is in electronic form. Otherwise theymust appear on printed covers that bracket the whole aggregate.

8. TRANSLATION

Translation is considered a kind of modification, so you may distribute translationsof the Document under the terms of section 4. Replacing Invariant Sections withtranslations requires special permission from their copyright holders, but you mayinclude translations of some or all Invariant Sections in addition to the original versionsof these Invariant Sections. You may include a translation of this License, and all thelicense notices in the Document, and any Warranty Disclaimers, provided that youalso include the original English version of this License and the original versions ofthose notices and disclaimers. In case of a disagreement between the translation andthe original version of this License or a notice or disclaimer, the original version willprevail.

If a section in the Document is Entitled “Acknowledgements”, “Dedications”, or “His-tory”, the requirement (section 4) to Preserve its Title (section 1) will typically requirechanging the actual title.

9. TERMINATION

You may not copy, modify, sublicense, or distribute the Document except as expresslyprovided under this License. Any attempt otherwise to copy, modify, sublicense, ordistribute it is void, and will automatically terminate your rights under this License.

However, if you cease all violation of this License, then your license from a particularcopyright holder is reinstated (a) provisionally, unless and until the copyright holderexplicitly and finally terminates your license, and (b) permanently, if the copyrightholder fails to notify you of the violation by some reasonable means prior to 60 daysafter the cessation.

Moreover, your license from a particular copyright holder is reinstated permanently ifthe copyright holder notifies you of the violation by some reasonable means, this is thefirst time you have received notice of violation of this License (for any work) from thatcopyright holder, and you cure the violation prior to 30 days after your receipt of thenotice.

Termination of your rights under this section does not terminate the licenses of partieswho have received copies or rights from you under this License. If your rights havebeen terminated and not permanently reinstated, receipt of a copy of some or all of thesame material does not give you any rights to use it.


10. FUTURE REVISIONS OF THIS LICENSEThe Free Software Foundation may publish new, revised versions of the GNU FreeDocumentation License from time to time. Such new versions will be similar in spiritto the present version, but may differ in detail to address new problems or concerns.See http://www.gnu.org/copyleft/.Each version of the License is given a distinguishing version number. If the Documentspecifies that a particular numbered version of this License “or any later version”applies to it, you have the option of following the terms and conditions either of thatspecified version or of any later version that has been published (not as a draft) bythe Free Software Foundation. If the Document does not specify a version number ofthis License, you may choose any version ever published (not as a draft) by the FreeSoftware Foundation. If the Document specifies that a proxy can decide which futureversions of this License can be used, that proxy’s public statement of acceptance of aversion permanently authorizes you to choose that version for the Document.

11. RELICENSING“Massive Multiauthor Collaboration Site” (or “MMC Site”) means any World WideWeb server that publishes copyrightable works and also provides prominent facilitiesfor anybody to edit those works. A public wiki that anybody can edit is an example ofsuch a server. A “Massive Multiauthor Collaboration” (or “MMC”) contained in thesite means any set of copyrightable works thus published on the MMC site.“CC-BY-SA” means the Creative Commons Attribution-Share Alike 3.0 license pub-lished by Creative Commons Corporation, a not-for-profit corporation with a principalplace of business in San Francisco, California, as well as future copyleft versions of thatlicense published by that same organization.“Incorporate” means to publish or republish a Document, in whole or in part, as partof another Document.An MMC is “eligible for relicensing” if it is licensed under this License, and if all worksthat were first published under this License somewhere other than this MMC, andsubsequently incorporated in whole or in part into the MMC, (1) had no cover textsor invariant sections, and (2) were thus incorporated prior to November 1, 2008.The operator of an MMC Site may republish an MMC contained in the site underCC-BY-SA on the same site at any time before August 1, 2009, provided the MMC iseligible for relicensing.

http://www.gnu.org/copyleft/

74 GNU libgomp

ADDENDUM: How to use this License for your documents

To use this License in a document you have written, include a copy of the License in thedocument and put the following copyright and license notices just after the title page:

Copyright (C) year your name.

Permission is granted to copy, distribute and/or modify this document

under the terms of the GNU Free Documentation License, Version 1.3

or any later version published by the Free Software Foundation;

with no Invariant Sections, no Front-Cover Texts, and no Back-Cover

Texts. A copy of the license is included in the section entitled ‘‘GNU

Free Documentation License’’.

If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts, replace the“with...Texts.” line with this:

with the Invariant Sections being list their titles, with

the Front-Cover Texts being list, and with the Back-Cover Texts

being list.

If you have Invariant Sections without Cover Texts, or some other combination of thethree, merge those two alternatives to suit the situation.

If your document contains nontrivial examples of program code, we recommend releasingthese examples in parallel under your choice of free software license, such as the GNUGeneral Public License, to permit their use in free software.

Funding Free Software 75

Funding Free Software

If you want to have more free software a few years from now, it makes sense for you tohelp encourage people to contribute funds for its development. The most effective approachknown is to encourage commercial redistributors to donate.

Users of free software systems can boost the pace of development by encouraging for-a-fee distributors to donate part of their selling price to free software developers—the FreeSoftware Foundation, and others.

The way to convince distributors to do this is to demand it and expect it from them. Sowhen you compare distributors, judge them partly by how much they give to free softwaredevelopment. Show distributors they must compete to be the one who gives the most.

To make this approach work, you must insist on numbers that you can compare, such as,“We will donate ten dollars to the Frobnitz project for each disk sold.” Don’t be satisfiedwith a vague promise, such as “A portion of the profits are donated,” since it doesn’t givea basis for comparison.

Even a precise fraction “of the profits from this disk” is not very meaningful, sincecreative accounting and unrelated business decisions can greatly alter what fraction of thesales price counts as profit. If the price you pay is $50, ten percent of the profit is probablyless than a dollar; it might be a few cents, or nothing at all.

Some redistributors do development work themselves. This is useful too; but to keepeveryone honest, you need to inquire how much they do, and what kind. Some kinds ofdevelopment make much more long-term difference than others. For example, maintaininga separate version of a program contributes very little; maintaining the standard versionof a program for the whole community contributes much. Easy new ports contribute little,since someone else would surely do them; difficult ports such as adding a new CPU to theGNU Compiler Collection contribute more; major new features or packages contribute themost.

By establishing the idea that supporting further development is “the proper thing todo” when distributing free software for a fee, we can assure a steady flow of resources intomaking more free software.

Copyright c© 1994 Free Software Foundation, Inc.Verbatim copying and redistribution of this section is permittedwithout royalty; alteration is not permitted.

Library Index 77

Library Index

EEnvironment Variable . . . . . . . 19, 20, 21, 22, 23, 24

FFDL, GNU Free Documentation License . . . . . . . 67

I

Implementation specific setting . . . . . 20, 22, 23, 24

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1